1. Field of the Invention
This invention relates generally to control of automotive vehicle dynamics, and more particularly to improvements in processing data from certain on-board sensors, including a lateral accelerometer, a steering wheel position sensor, and a yaw rate sensor, for accomplishing robust control in the presence of certain input disturbances, such as those due to road surface banking.
2. Background Information
Certain motion sensors, such as steering wheel position sensors, yaw rate sensors, and lateral accelerometers (lateral acceleration sensors), provide useful inputs to an automotive vehicle dynamic control system. Such a control system utilizes a microprocessor that processes data, including input signal measurements received from such sensors, to cause adjustments in and/or actuations of a dynamic control system. The responses of such sensors can be sufficiently fast in relation to vehicle motion that their signal measurements can very closely track changing values of the respective measured parameters in real time. Accordingly, a microprocessor-based control system which receives such signal measurements as inputs can quickly process them to perform desired control action, such as performing a control function that strives to correct for rapidly changing vehicle dynamics via an appropriate system of the vehicle.
The electric signal measurements of such sensors may be subject to D.C. bias, or offset, which can influence measurement accuracy. Two identifiable and independent causes of such bias, or offset, in a lateral acceleration sensor are electrical drift and road bank angle disturbance. One technique previously proposed for compensating a system with respect to such bias, or offset, involves adjustment by long-term filtering/averaging of the signal measurement; another involves adjustment based on functional redundancy with other sensors using recent (short-term) measurements from those other sensors. The "long-term" filtering/averaging methodology is robust to dynamic vehicle maneuvers, but may have difficulty in promptly tracking D.C. electrical drift. The "short-term" methodology based on sensor functional redundancy may be better at tracking D.C. electrical drift, but may be overly sensitive to dynamic vehicle maneuvers.
The inventors of the present invention have realized that those two prior techniques do not distinguish times when the particular compensation that they provide should be made from times when it should not. Hence, when such known techniques are embodied in a vehicle dynamic control system, they may, at certain times, furnish a response, or lack thereof, that causes the system to respond, or not respond, in a manner other than what may be considered the most desirable, given the nature of events prompting the performance of such techniques.
When a vehicle is traveling along a road that is banked, for example because the road surface has a crown, the driver will steer the vehicle so as to inherently compensate for the road bank angle. Because of the presence of road banking, the driver is actually manipulating the steering wheel to positions slightly different from positions that he would in the absence of road banking, often without consciously realizing that he is doing so.
Because the steering angle sensed by a steering wheel angle sensor is uninfluenced by road banking, the electric signal measurement which it provides will exactly correlate with actual straightline travel of the vehicle at only one road bank angle. For example, if a particular signal measurement correlates to straightline vehicle travel when the road bank angle is zero, that specific signal measurement will depart from indicating straightline vehicle ravel when the vehicle is traveling along a banked road because the driver will have to readjust the steering angle to compensate for the road bank. The extent to which that specific steering angle signal measurement deviates from representing true straightline vehicle travel increases as the road bank angle increases.
In certain driving situations, for example when a vehicle is traveling at a banked angle due to the nature of an underlying road surface, each of an electric signal measurement of steering angle provided by a steering angle sensor and an electric signal measurement of vehicle lateral acceleration provided by a lateral accelerometer may have a respective adaptive offset that is different from the corresponding absolute offset. Absolute offset means the offset existing in the absence of any disturbances (both electrical and mechanical) to the sensor, and such absolute offset may in fact be zero in value. In the example of a vehicle traveling along a crowned road at a banked angle, the adaptive steering wheel center (offset) may be significantly different from its mechanical center while the adaptive offset in the signal measurement of a lateral accelerometer may be significantly different from its electrical D.C. offset.